1. Field of the Invention
This invention relates to a method and means of preventing corrosion in a steam boiler system. More particularly, this invention relates to an improved, efficient method and means of preventing corrosion due to condensation of flue gas within the fuel economizer and within the exhaust stack of the steam boiler system.
2. Description of the Prior Art
Fuel economizers, or similar heat recovery devices, are well-known devices used to reduce the amount of fuel necessary to convert feedwater into steam in boiler systems by extracting waste heat from flue gas. Because of the energy crisis, industry needs the fuel savings that economizers can offer.
One problem, however, with the use of a fuel economizer in a boiler system is that corrosive acids can condense out of the flue gas inside the economizer if the temperature of the flue gas from the boiler drops below certain dew points. Condensation of acid vapors from the flue gas is the result of the flue gas contacting the metal heating surfaces in the boiler economizer. Since the temperature of the metal heating surfaces (the tube wall and the related finning) in contact with the flue gas is more nearly that of the feedwater than the flue gas, there are two items that should be analyzed; first, the sulfur content of the fuel being burned and, second, the temperature of the feedwater at the inlet of the economizer.
Another problem that this invention addresses is exhaust stack corrosion. The temperature of the metal, or stack wall, in contact with the exit flue gas determines the extent of acid corrosion due to condensation.
One way to reduce corrosion is to use metals with a greater amount of corrosion resistance. This may, however, be expensive and not totally effective. Another solution that has been tried is to regulate the amount of feedwater flowing through the economizer and bypassing the economizer at times in response to a temperature control valve. Such a system has been attempted in Merrit, Jr., U.S. Pat. No. 3,910,236. Bypassing feedwater at low load conditions creates a very real potential for steam formation within the economizer tubing which is not designed to be a steam generator. More importantly, bypassing feedwater at low load conditions does not prevent corrosion near the inlet of the economizer since the tube metal temperature determines the extent of acid corrosion and the feedwater temperature determines the tube metal temperature.
Therefore, it is a principal object and purpose of this invention to provide a method of preventing corrosion within both the boiler economizer and the exhaust stack of the boiler system using a thermostatically controlled feedwater preheat system.